The invention concerns a self-cleaning nozzle particularly suitable for use in a plurality in a spray apparatus for the application of a fluid, such as a liquid suspension of starch, binder, adhesive, colorant or other material such as a surface coating agent, onto at least one surface of a paper web in a papermaking process.
In the manufacture of paper, board and similar cellulosic products, a fluid stock consisting of from about 1% solids suspended in about 99% water is ejected at high speed and precision from a headbox slice onto a moving forming fabric, or between two fabrics, in the forming section of a papermaking machine. The stock is drained through the fabric or fabrics by gravity and/or vacuum so that, by the end of the forming section, a cohesive nascent web of fibers is provided. This web is then transferred to a downstream press section where further water removal occurs by mechanical means as the web, together with one or more press fabrics is passed through at least one, and usually a series, of nips formed between pairs of rotating press rolls so as to remove a further portion of the water entrained in the web. At the end of the press section, the web is transferred to the dryer section where its remaining moisture is removed by evaporative means as it is passed, together with one or more dryer fabrics, over a series of steam heated rotating drums known as dryer cans or cylinders.
The paper product thus obtained will usually require at least one or more subsequent chemical or physical treatments so as to render it suitable for its intended use and impart to it various properties, such as smoothness, gloss, impermeability, rigidity, color, and so on, as desired. These properties are often obtained by applying a surface sizing agent or other material (such as a colorant, optical brightener, or water resistant film or other coating) during or following drying. This is frequently done by passing the sheet through a pond sizer so that it is immersed in the desired solution, or by applying size as a film using a film sizing apparatus as the sheet passes through a nip. In addition, it is often necessary to apply water onto the sheet so as to improve the uniformity of the moisture content across the full width of the manufactured web.
A wide variety of both pond and film sizing application devices are available on the market today, and numerous patents cover various aspects of their technology. Although suitable for use in certain applications, the known devices are limited in machine speed potential and cannot exceed these limits without causing process instabilities, or web breaks to occur due to strength losses and/or absorbency variations in the web that is delivered to the sizing apparatus. It is also difficult to precisely control the average amount of material applied to the sheet independently of machine speed with the known devices, and the specific amount applied at different locations across the full width of the manufactured web. As well, the known devices are difficult to keep clean.
It has been found that one means of overcoming at least a portion of the aforementioned problems of the known film or pond coating methods is to spray the desired process liquid directly on to the sheet as it passes beneath or through one or more arrays of spray nozzles. Both the average amount and the cross-directional uniformity of spray application are less dependent on sheet properties than by conventional application means, and it is also possible to use relatively high concentrations of suspended or dissolved materials in the process liquid. In addition, a spray apparatus allows for more precise control of the amount, and type, of materials to be delivered as the liquid and solids concentration provided to at least a portion of the nozzles can be proportioned to allow for a somewhat profiled delivery to the sheet. However, a problem common to the known spray apparatuses is that it is difficult to keep the nozzle areas clean and free of contaminants, particularly where a sizing material is being applied. Typically, the solids in the process liquid will become deposited proximate the nozzle tip, and their build up will eventually disrupt the spray pattern and clog the nozzle outlet.
Nozzles for spraying a dispersed mist onto a moving web, and arrangements of such nozzles, are well known, and have been described, for example, by Sundholm et al. EP 435904 and EP 682571; Kangas et al. U.S. Pat. No. 6,866,207 and U.S. Pat. No. 6,969,012; and Diebel et al. EP 2 223 748. Others are known and used.
Tynkkynen et al. EP 2 647 760 describes a nozzle in which the tip or end is provided with means for controlling its temperature so as to prevent or at least minimize the adherence of undesirable matter from the fluid spray that is applied to the moving web. However, this is a high pressure type nozzle with a small tip opening, and the solution proposed in the disclosure is not appropriate to nozzles having a relatively larger spray opening at the tip, where the process liquid is dispersed by a flow of pressurized air.
None of the known prior art effectively addresses the issue of preventing deposits of the sprayed material and/or contaminants being formed around the nozzle discharge outlet that affects the spray dispersion quality as well as the spray pattern.